It is estimated that when the weight of a vehicle is decreased by 10%, its fuel consumption will be lowered by 5%-8%, and the emission of greenhouse gas CO2 and such pollutants as NOx, SO2, etc., will be reduced as well. Self-owned brand passenger vehicles in China are appropriately 10% heavier than their foreign counterparts, and the difference is even larger for commercial vehicles. As the main raw material of an automobile body, automobile steel plate accounts for about 60-70% of the weight of the automobile body. Massive use of high-strength and super-high-strength steel plates with strength at the level of 590-1500 MPa instead of traditional automobile steel is an optimal solution to the problem of material in order to achieve “reduced weight, less energy consumption, higher safety and lower manufacturing cost” for automobiles, and it is of great significance for the building of low-carbon society. Hence, it has been a trend in recent years for the development of steel plates to enhance the strength of the steel plates so that the thickness of the steel plates can be reduced. Development and application of advanced high-strength automobile steel mainly strengthened by phase change has been one of the mainstream subjects under research in various large steel companies around the world.
The high strength of traditional super-high-strength steel is originated from the high-strength phase structure of martensite, bainite, etc., but the plasticity and the formability are reduced significantly at the same time. Introduction of a certain amount of residual austenite into the structure of martensite or bainite is an effective technical approach to obtain high-strength and high-plasticity materials. For example, TRIP steel is composed of ferrite, bainite and residual austenite, and has relatively high strength and plasticity, but this phase structure restricts further improvement of its strength. Thus, replacement of bainite by martensite as the main strengthening phase has begun to gain attention. On the other hand, hot-dip galvanized products are used in automobiles in large quantities, up to 80% on average, and even up to 100% for some types of automobiles, due to their much better rust resistance than common cold rolled products. The development of high-strength, hot-dip galvanized steel plates starts late in China, and a full range of such steel plates are still not available. Especially, there remains a domestic blank for high-strength, hot-dip galvanized steel products exhibiting a strength of greater than 1000 MPa, superior formability and low cost, particularly for hot-dip galvanized steel products with high Si content design due to their metallurgical nature.
Japanese Patent Application JP2010-053020 discloses a high-strength, hot-dip galvanized steel plate with superior processability and a method for manufacturing the same, wherein the composition comprises, based on mass percentage, C: 0.04˜0.15%, Si: 0.7˜2.3%, Mn: 0.8˜2.2%, P<0.1%, S<0.01%, Al<0.1%, N<0.008%, and the balance of iron and unavoidable impurities. The structure comprises 70% or higher ferrite phase, 2˜10% bainite phase, 0˜12% pearlite phase and 1˜8% residual austenite phase. The average grain size of the ferrite is 18 μm or less, and that of the residual austenite is 2 μm or less. This inventive steel possesses a tensile strength of 590 MPa or higher and good processability (ductility and pore expandability). However, the steel of this invention is a TRIP steel with a tensile strength at the level of 600-700 MPa which can not meet the requirement of super-high-strength steel.
Chinese Patent Application CN200810119822 discloses a cold rolled, hot-dip galvanized dual phase steel at the level of 1000 MPa and a method for manufacturing the same, and pertains to the technical field of high-strength steel plates for cold rolling and galvanization, wherein the composition comprises, based on mass percentage, C: 0.06˜0.18%, Si: ≤0.1%, Mn: 1.2˜2.5%, Mo: 0.05˜0.5%, Cr: 0.05˜0.6%, Al: 0.005˜0.05%, Nb: 0.01˜0.06%, Ti: 0.01˜0.05%, P≤0.02%, S≤0.01%, N≤0.005%, and the balance of iron and unavoidable impurities. According to this invention, Si is replaced with Cr and Mo to enlarge the dual phase area of austenite+ferrite and improve the hardenability of the dual phase steel. Meanwhile, the grains are refined by addition of alloy elements Nb, Ti to increase the strength and toughness of the steel, and impart good weldability and applicability to the steel. The strength may reach the level of 1000 MPa or higher, which can satisfy the performance requirements of super-high-strength cold rolled, hot-dip galvanized steel useful for automobiles. Nevertheless, this inventive steel has an elongation of only about 10% which cannot meet the high formability requirement of super-high-strength steel useful for automobiles. Moreover, this inventive steel requires addition of rather large amount of expensive alloy elements such as Mo, Cr, Nb, Ti, etc, rendering it unsuitable for automobile steel which needs very strict cost control.
Japanese Patent Application JP 2008-255442 discloses a super-high-strength, hot-dip galvanized steel having a tensile strength of 780 MPa or higher and a method for manufacturing the same, wherein the composition comprises, based on mass percentage, C: 0.03˜0.25%, Si: 0.02˜0.60%, Mn: 2.0˜4.0%, Al≤0.8%, N: 0.0020˜0.015%, Ti≤0.5%, Nb≤0.5%, Ti+Nb: ≥0.05%, Si: 0.02˜1.00%, Cu≤1.5%, Ni≤1.5%, Cu+Ni: ≥0.05%, and the balance of iron and unavoidable impurities. The microstructure is consisted of ferrite having an average grain size of less than 5 μm and a hard second phase having an average grain size of less than 0.5 μm. The manufacture method according to this invention comprises the following steps: cooling the steel plate to 700° C. or lower within 10 seconds right after hot rolling; taking up the steel plate at a temperature between 400 and 700° C.; acid-pickling the steel plate; cold-rolling the steel plate at a rolling reduction of 35˜80%; annealing the steel plate at a temperature between Ac3˜950° C. for 5˜200 seconds; cooling the steel plate to a temperature between 400 and 600° C. and holding at this temperature for 5 to 200 seconds; hot-dip galvanizing the steel plate; and subjecting the galvanized steel plate to alloying treatment at 540° C. or lower. This invention can realize a tensile strength of 1000 MPa and an elongation up to 18%, and thus the performance requirements of super-high-strength automobile steel are met. However, this invention entails addition of a considerable amount of alloy elements Nb, V, Ti (total amount>0.25%), which not only increases the material cost greatly, but also adds to the difficulty in casting, hot rolling and the like in the manufacture process.